The present invention relates to methods and apparatus for making elongated structures of silicon and silicon carbide, and more particularly structures having at least one flat side.
It has been known for many years that vapor deposited silicon carbide structures such as rods and tubes can be made by vapor depositing silicon carbide on a heated form of material capable of withstanding the deposition temperatures. For example, U.S. Pat. No. 3,157,541 (Heywang et al.), U.S. Pat. No. 3,011,877 (Schweickert et al.), U.S. Pat. No. 3,350,182 (Hunter et al.), and U.S. Pat. No. 3,397,448 (Tucker) all describe methods of this sort. Various other examples exist in the prior art. Silicon carbide structures are particularly useful in atmospheres where high temperatures or chemical inertness are desirable characteristics.
More recently it has been suggested that semiconductor grade silicon be formed in tubular shapes for use, for example, as furnace tubes in the processing of semiconductor materials. The silicon tubes have been made in much the same manner as the previously suggested methods for producing silicon carbide tubes, i.e., depositing silicon from the vapor phase on a heated rod of compatible material. The aforementioned Schweickert et al. patent describes deposition of silicon on a heated carrier from silicon containing gases as does U.S. Pat. No. 2,999,735 (Reuschel). This method for the deposition of silicon upon a heated carrier has become virtually the standard method for production of semiconductor grade silicon both in the United States and abroad.
The manufacture of furnace tubes of silicon and silicon carbide is generally made using graphite as the substrate material for deposition. After deposition to the desired wall thickness the graphite is removed and in most cases is not reusable. Accordingly, for purposes of economy the cheapest possible form of graphite suitable for the purpose is desirable. It has been found that there are advantages in using furnace tube configurations which have at least a flat bottom portion or which have a rectangular cross section. This has been accomplished by machining of either the graphite form or the finished tube both of which result in substantial waste of materials and time.